1. Technical Field
The present invention relates to an automatic document feeder, an image reading device, and an image forming apparatus. More particularly, the present invention relates to an automatic document feeder that is of a sheet-through type applicable for reading double face original documents, an image reading device that includes such automatic document feeder, and an image forming apparatus that includes such automatic document feeder.
2. Discussion of the Related Art
Related art automatic document feeders are provided to an image reading device and an image forming apparatus, for example, a copier, facsimile machine, printer, multifunctional machine, and so forth.
Automatic document feeder (hereinafter, referred to as an “ADF”) of a sheet-through type has been widely used in recent years.
In such sheet-through type ADF, a double face original document that is an original document having images on both faces is transferred to a scanning position so as to read the image formed on the front face thereof. After the image on the front face thereof is read, the original document is switched back to the scanning position to read the image formed on the reverse face thereof. Then, the original document is switched back again to the scanning position to collate the pages of the original document in a proper order.
To enhance productivity in processing a plurality of double face original documents including a first or preceding original document and a second or following original document, the above-described sheet-through type ADF conducts a document transfer control in which the front face of the second original document is read by passing over the scanning position for the first time before the first original document passes over the scanning position for the third time. By transferring two double face original documents in a substantially concurrent manner in a document transfer path in the ADF, the document transfer control conducted by the ADF can enhance its productivity when handling a plurality of double face original documents.
There are some techniques to enhance productivity in processing double face original documents.
A related art ADF with one of the techniques includes a circulating and reversing passage and a control unit. The circulating and reversing passage includes a scanning position for reading images formed on both front and reverse faces of an original document. The control unit picks up the second original document from an original document stacker before the trailing edge of the first original document passes over the scanning position, at least when causing the first original document after once passing over the scanning position to pass over the scanning position for the second time via the circulating and reversing passage. With the above-described structure, when two original documents are transferred in the ADF in a substantially concurrent manner, the second original document is transferred in an overlaying manner with the first original document in the same document transfer path while the first original document is being held and stopped by a pair of registration rollers.
However, with the above-described operation, the first and second original documents may rub each other. This can cause an adhesion of dirt or a contamination on these original documents and a decrease of the document transfer ability, which may result in an occurrence of jams.
To eliminate such drawbacks, a related art ADF of a sheet through type with another technique includes an automatic document carrying device, a pair of switchback members, and a control unit.
The automatic document carrying device separates and feeds original documents one by one from an original document stacker and switches back the separated original document to transfer to a scanning position.
The pair of switchback members is disposed below the original document stacker, in a switchback path connecting to a reverse path for reversing the read original document. The pair of switchback members that includes a normal and reverse rotation drive roller and a driven roller can rotate in normal and reverse rotations, and the driven roller can separate from and contact to the normal and reverse rotation drive roller.
The control unit controls to drive the automatic document carrying device and the pair of switchback members.
The related art ADF having such structure causes the pair of switchback members to sandwich the first original document, and to separate the driven roller from the normal and reverse rotation drive roller while the first original document is sandwiched by the pair of switchback members. By separating the pair of switchback members, the second original document is transferred to the switchback path. Then, in a condition in which the first and second original documents are overlapped, the driven roller of the pair of switchback members contacts the normal and reverse rotation drive roller again so as to sandwich the first and second original documents while a part of the first original document and a part of the second original document are being overlaid.
In the above-described related art ADF, two original documents are carried and transferred in the document transfer path in the ADF in a constantly sequential manner. Therefore, the two original documents keep a constant distance.
With the above-described structure, when the related art ADF discharges the two original documents, the first original document after being collated in a proper page order is discharged at the transfer speed according to a read scan magnification or a magnification for a read scan of an image.
It is noted that the lower the read scan magnification is set, the greater the reduction ratio becomes, and the faster the transfer speed of the first original document becomes.
Therefore, when the read scan magnification is set to a relatively low ratio, the transfer speed of the first original document is increased by the level of magnification, which can cause the original document jump to a discharging tray.
In other words, by accounting for the stackability of discharged original documents, a document transfer control for double face original documents cannot be conducted with a ratio exceeding a constant read scan magnification.